Porous carbon, and positive electrode and lithium secondary battery comprising same

ABSTRACT

Porous carbon particles, and a positive electrode active material and a lithium secondary battery including the same. This may improve the energy density of the lithium secondary battery by applying a porous electrode containing micropores and mesopores and having a uniform size distribution and shape as a positive electrode material.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Continuation of U.S. application Ser. No.16/646,473, filed on Mar. 11, 2020, which is the U.S. National Phaseunder 35 U.S.C. § 371 of International Application No.PCT/KR2018/012440, filed on Oct. 19, 2018, which claims the priorityunder 35 U.S.C. § 119(a) to Korean Application No. 10-2017-0147765,filed on Nov. 8, 2017, all of which are hereby expressly incorporated byreference into the present application.

TECHNICAL FIELD

The present application claims the benefit of Korean Patent ApplicationNo. 10-2017-0147765 filed on Nov. 8, 2017, all the contents of which areincorporated herein by reference.

The present invention relates to a porous carbon capable of improvingthe energy density of a battery by acting as a sulfur carrier in apositive electrode of a lithium secondary battery, for example, alithium-sulfur secondary battery, and a positive electrode and a lithiumsecondary battery comprising the same.

BACKGROUND ART

The present invention relates to porous carbon, and a positive electrodeactive material, and a lithium secondary battery comprising the same.More specifically, the present invention can improve the energy densityof the lithium secondary battery by applying a porous electrodecontaining micropores and mesopores and having a uniform sizedistribution and shape as a positive electrode material.

Until a recent date, there has been considerable interest in developingbatteries with high energy densities using lithium as a negativeelectrode. For example, as compared to other electrochemical systemswith a lithium inserted carbon negative electrode and a nickel orcadmium electrode that reduce the energy density of the battery byincreasing the weight and volume of the negative electrode due to thepresence of the non-electroactive material, since lithium metal has lowweight and high capacity characteristics, lithium metal has attractedmuch attention as a negative electrode active material forelectrochemical batteries. Lithium metal negative electrode, or negativeelectrodes, which mainly comprise lithium metal, provide the opportunityto construct a battery that is lighter and has a higher energy densitythan the battery such as a lithium-ion, nickel metal hydride ornickel-cadmium battery. These features are highly desirable forbatteries for portable electronic devices, such as cell phones andlap-top computers, where premiums are paid with low weighted value.

Positive electrode active materials of these types for lithium batteriesare known and comprise a sulfur-containing positive electrode activematerial containing a sulfur-sulfide bond, and achieve high energycapacity and rechargeability from electrochemical cleavage (reduction)and reforming (oxidation) of sulfur-sulfur bonds.

Since there are advantages that the lithium-sulfur secondary batteryusing lithium and alkali metal as a negative electrode active materialand sulfur as a positive electrode active material as described abovehas theoretical energy density of 2,800 Wh/kg and theoretical capacityof sulfur of 1,675 mAh/g, which is much higher than other batterysystems, and sulfur is rich in resources, is cheap and is anenvironmentally friendly substance, the lithium-sulfur secondary batteryis attracting attention as a portable electronic device.

However, there were problems that since sulfur used as a positiveelectrode active material of the lithium-sulfur secondary battery isnonconductor, it is difficult to transfer electrons generated byelectrochemical reaction and that the life characteristics and ratecharacteristics of the battery are inhibited due to the leaching issueof poly sulfide (Li₂S₈˜Li₂S₄) during charging/discharging and the slowkinetic of electrochemical reactions by the low electrical conductivityof sulfur and lithium sulfide (Li₂S₂/Li₂S).

In this regard, recently, in order to solve the leaching issue of polysulfide during charging/discharging of the lithium-sulfur secondarybattery and the low electrical conductivity of sulfur and lithiumsulfide, a carbon material of the porous structure with high electricalconductivity is used as a sulfur carrier.

Japanese Laid-open Patent Publication No. 2013-125697 discloses a porouscarbon having a pore, which is a conductive material that can becompounded with an active material as an electrode material. The porouscarbon can be compounded with a compound containing sulfur and/or sulfuratom, and is capable of improving the electron conductivity of theelectrode material. Specifically, the pore capacity of the conductivematerial is 0.5 cc/g or more and 4.0 g/cc or less, the pore diameter is100 nm or less, and the particle diameter of the conductive material is1 nm or more and 500 μm or less.

There have been many reports on the application of such porous carbon inconventional lithium-sulfur secondary batteries, but there is still alimit to the improvement of energy density per unit volume and unitvolume.

PRIOR ART DOCUMENT Patent Document

Japanese Laid-open Patent Publication No. 2013-125697, “Composition,electrode and battery comprising lithium particles.”

DISCLOSURE Technical Problem

As a result of various studies to solve the above problems, theinventors of the present invention have found that the specific surfacearea of a porous carbon is maintained at a level equal to or greaterthan that of the conventional porous carbon while increasing the overallpore volume and improving the battery performance, by preparing theporous carbon which may act as a sulfur carrier in a positive electrodeof a lithium-sulfur secondary battery wherein the porous carbon isprepared to have a mixed pore structure of micropores and mesopores anda uniform particle shape and size.

Therefore, it is an object of the present invention to provide a porouscarbon which may be used as a sulfur carrier in a positive electrode ofa lithium-sulfur secondary battery and a method for preparing the same.

Also, it is another object of the present invention to provide apositive electrode active material comprising the porous carbon and amethod of preparing the same.

Also, it is further another object of the present invention to provide alithium secondary battery comprising such a positive electrode activematerial.

Technical Solution

In order to achieve the above objects, the present invention provides aporous carbon comprising micropores having a diameter of 1 nm to 8 nmand mesopores having a diameter of 20 nm to 40 nm wherein the porouscarbon is a spherical particle having a particle diameter of 2 μm to 10μm.

The porous carbon may comprise the micropores and the mesopores in apore volume ratio of 1:40 to 50.

The volume of the mesopores may be 3.5 cm²/g and more.

The porous carbon may have a specific surface area of 1000 cm²/g to 1300cm²/g.

The present invention also provides a method for preparing the porouscarbon comprising the steps of: (S1) dissolving porous silica in anorganic solvent and mixing with a hydrate for introducing an Al acidsite to prepare a solution of the porous silica; (S2) evaporating anorganic solvent in the porous silica solution to obtain porous silicaparticles; (S3) subjecting the porous silica particles to the firstheat-treatment to obtain porous silica particles having an Al acid siteintroduced therein; (S4) impregnating the pores of the porous silicaparticles having the Al acid site introduced therein with a carbonprecursor and then subjecting it to the second heat treatment to obtaina carbon-silica composite; and (S5) etching the silica in thecarbon-silica composite to obtain porous carbon.

The hydrate for introducing the Al acid site may be aluminum chloridehexahydrate.

The first heat treatment may be performed by raising the temperature to500° C. to 600° C. at a rate of 0.5° C./min to 3° C./min.

The carbon precursor may be at least one selected from the groupconsisting of furfuryl alcohol, sucrose and glucose.

The second heat treatment may be carried out at 70° C. to 100° C. for 7hours to 10 hours.

After the second heat treatment, the method may further comprise a stepof raising the temperature at a rate of 0.5° C./min to 3° C./min underan inert atmosphere, and then performing the third heat treatment at700° C. to 1000° C. for 1 hour to 5 hours.

The etching solution used in the etching may be a solution comprising atleast one selected from the group consisting of hydrofluoric acid (HF),hydrogen peroxide (H₂O₂), nitric acid (HNO₃) and potassium hydroxide(KOH).

The present invention also provides a porous carbon, and a positiveelectrode active material comprising a sulfur-containing materialcarried within pores of the porous carbon.

The content of sulfur carried in the porous carbon may be 50 to 80 wt. %based on the total weight of the positive electrode active material.

The present invention also provides a method for preparing a positiveelectrode active material, comprising the steps of: (P1) forming a mixedpowder of the porous carbon and a sulfur-containing material; (P2)mixing the mixed powder with a solvent for dissolving sulfur to form amixture; and (P3) subjecting the mixture to heat treatment under vacuumto impregnate the pores of the porous carbon with sulfur.

The solvent for dissolving sulfur may be at least one selected from thegroup consisting of CS₂, ethylenediamine, acetone and ethanol.

The present invention also provides a positive electrode for a lithiumsecondary battery comprising the positive electrode active material.

The present invention also provides a lithium secondary batterycomprising the positive electrode.

Advantageous Effects

Since the porous carbon according to the present invention comprisesmicropores and mesopores having different sizes, when the porous carbonis applied as a positive electrode material for a lithium secondarybattery, for example, a lithium-sulfur secondary battery, the specificsurface area is improved by the micropores, and thus the batteryperformance may be improved and the energy density of the battery may beimproved by increasing the loading of sulfur by the mesopores. Inaddition, since the mesopores provide a sufficient pore volume, it ispossible to maximize the participation of sulfur in the oxidation andreduction reaction by providing a space capable of facilitating theentry and exit of the electrolyte solution while carrying the sulfur.

In addition, since the porous carbon according to the present inventionhas a uniform spherical shape and a uniform size, when applied as apositive electrode active material for a lithium-sulfur secondarybattery, the porous carbon may improve the packing density of thepositive electrode active material on the current collector to improvethe energy density of the battery.

DESCRIPTION OF DRAWINGS

FIGS. 1a and 1b are SEM (scanning electron microscope) photographs ofporous silica synthesized in Preparation Example 1.

FIGS. 2a to 2c are SEM (scanning electron microscope) and TEM(transmission electron microscope) photographs of the porous silicaprepared in Preparation Example 1.

FIGS. 3a to 3d are graphs of the isotherm linear plot and the porediameter distribution as a result of nitrogen adsorption/desorptionanalysis of the porous carbon according to Examples 1 to 4,respectively.

FIG. 3e are SEM (scanning electron microscope) and TEM (transmissionelectron microscope) photographs of the porous silica according toExample 5.

FIG. 4 is a graph showing the results of nitrogen adsorption/desorptionanalysis on the porous carbon of Example 1 and the activated carbon ofComparative Example 1.

FIG. 5 is a graph showing voltage profiles depending on capacities inlithium-sulfur secondary batteries of Example 1 and Comparative Example1.

FIG. 6 is a graph showing the results of galvanostaticcharging-discharging analysis for lithium-sulfur secondary batteries ofExample 1 and Comparative Example 1.

FIG. 7 is a graph showing discharging capacity of the lithium-sulfursecondary batteries of Example 1 and Comparative Example 1 depending onthe number of cycles during high-rate discharging.

BEST MODE

Hereinafter, the present invention will be described in detail in orderto facilitate understanding of the present invention.

The terms and words used in the present specification and claims shouldnot be construed as limited to ordinary or dictionary terms, and shouldbe construed in a sense and concept consistent with the technical ideaof the present invention, based on the principle that the inventor canproperly define the concept of a term to describe his invention in thebest way possible.

Porous Carbon

The present invention relates to a porous carbon which may be used as apositive electrode material for a lithium secondary battery.

The porous carbon according to the present invention comprisesmicropores and mesopores having different sizes and is characterized byhaving a uniform particle size and shape.

In the present invention, the porous carbon comprises micropores andmesopores having different sizes. Hereinafter, the size of the porescomprised in the porous carbon means the diameter of the pores.

The micropores not only increase the specific surface area of the porouscarbon but also have an effect of carrying sulfur and thus inhibitingthe release of poly sulfide. The diameter of the micropores may be 1 nmto 8 nm. If the diameter of the micropore is less than the above range,the pores are excessively small, so that the pore may be easily cloggedin the sulfur bearing process. If the diameter of the micropore islarger than the above range, the effect of increasing the specificsurface area of the porous carbon may be insignificant.

Since the mesopore is larger in pore size than the micropore and mayplay a sulfur bearing role capable of carrying more sulfur, the mesoporemay improve the energy density of the battery by increasing the sulfurcontent in the positive electrode of the lithium secondary battery, forexample, the lithium-sulfur secondary battery. Also, due to themesopore, the entry and exit of the electrolyte solution in the positiveelectrode of the lithium-sulfur secondary battery is facilitated and theleaching issue of poly sulfide may be improved by adsorption.

The mesopore may have a diameter of 20 nm to 40 nm. If the diameter ofthe mesopore is less than the above range, the loading of the sulfur inthe mesopore is reduced, the entry and exit of the electrolyte solutionis not easy, the space for adsorbing the poly sulfide is insufficient,and thus the leaching issue of the poly sulfide cannot be solved. If thediameter of the mesopore exceeds the above range, the pore size becomesexcessively large, and thus the leaching issue of the polysulfide in thepositive electrode becomes serious, and the durability of the electrodemay be reduced.

Also, the pore volume of the mesopore may be 3.5 cm²/g or more,preferably 3.5 cm²/g to 4.5 cm²/g, and more preferably 3.8 cm²/g to 4.2cm²/g. If the volume of the mesopore is less than the above range, theloading of the sulfur in the pores is reduced, and the effect ofimproving the energy density of the battery is insignificant. If thevolume of the mesopore exceeds the above range, the loading of sulfur isimproved to increase the sulfur content in the electrode and thusimprove the energy density, but the mechanical strength of the carbonstructure is relatively decreased, and the durability of thesulfur-carbon composite and the electrode may be deteriorated during theslurry preparing process.

In the porous carbon according to the present invention, the microporeand the mesopore may be contained at a pore volume ratio of 1:20 to 70,preferably 1:30 to 60, more preferably 1:40 to 50. If the pore volumeratio of the mesopores to the micropores is less than the above range,the specific surface area may be improved, the loading of the sulfur isreduced, and this the effect of improving the energy density of thebattery is insignificant. If the pore volume ratio of the mesopore tothe micropore exceeds the range, the loading of sulfur is increased, butthe proportion of mesopores is relatively high and thus the specificsurface area may be reduced.

The specific surface area of the porous carbon according to the presentinvention may range from 1000 m²/g to 1300 m²/g, preferably from 1150m²/g to 1300 m²/g, and more preferably from 1200 m²/g to 1300 m²/g. Ifthe specific surface area of the porous carbon is less than the aboverange, the discharging capacity may be lowered. If the specific surfacearea of the porous carbon is more than the above range, the case is acase when there are relatively many micro pores, and thus the loading ofsulfur may be reduced and the energy density of the battery may bereduced.

Also, since the porous carbon has a uniform shape and size, when appliedas a material for a positive electrode material, for example, a positiveelectrode active material, the packing density of the positive electrodeactive material on the current collector may be improved.

Specifically, the porous carbon has a spherical uniform shape and has auniform size with a particle diameter of 2 μm to 10 μm, preferably 3 μmto 7 μm, more preferably 4 μm to 6 μm. If the particle diameter of theporous carbon is less than the above range, the loading of the sulfurmay be reduced. If the particle diameter of the porous carbon exceedsthe above range, the packing density of the positive electrode activematerial on the current collector may be lowered.

Preparing Method of Porous Carbon

The present invention also relates to a method for preparing a porouscarbon which may be used as a positive electrode material for a lithiumsecondary battery, comprising the steps of (S1) dissolving porous silicain an organic solvent and mixing with aluminum chloride hexahydrate toprepare a solution of the porous silica; (S2) evaporating an organicsolvent in the porous silica solution to obtain porous silica particles;(S3) subjecting the porous silica particles to the first heat-treatmentto obtain porous silica particles having an Al acid site introducedtherein; (S4) impregnating the pores of the porous silica particleshaving the Al acid site introduced therein with a carbon precursor andthen subjecting it to the second heat treatment to obtain acarbon-silica composite; and (S5) etching the silica in thecarbon-silica composite to obtain porous carbon.

Hereinafter, the method for preparing the porous carbon according to thepresent invention will be described in more detail for each step.

Step (S1)

In step (S1), a porous silica solution may be prepared by dissolving aporous silica in an organic solvent and mixing it with a hydrate forintroducing an Al acid site.

In the present invention, the porous silica plays a role of a templatefor synthesizing the porous carbon. When porous silica having a particlesize of 2 μm to 10 μm is used, it may be advantageous to synthesizeporous carbon having uniform shape and size.

The organic solvent may be at least one selected from the groupconsisting of ethanol, methanol, propanol, butanol, ethyl acetate,chloroform, and hexane and is not limited as long as it is an organicsolvent capable of dissolving porous silica.

The hydrate for introducing the Al acid site may be aluminum chloridehexahydrate and is used to introduce the Al acid site into the poroussilica.

The porous silica solution in step (S1) may be prepared by using 1 to 5parts by weight of the porous silica and 0.21 to 1.05 parts by weight ofthe hydrate for introducing the Al acid site, relative to 100 parts byweight of the organic solvent.

If the amount of the porous silica is less than 1 part by weight, theyield of porous carbon produced is lowered and the relative ratio of theacid sites is increased, and thus there may be a restriction on thecarbonization reaction. If the amount of the porous silica exceeds 5parts by weight, the relative ratio of the acid sites may be lowered andthus the polymerization of the carbon precursor for the synthesisreaction of the porous carbon may be difficult to proceed.

If the amount of the hydrate for introducing the Al acid site is lessthan 0.21 part by weight, the acid sites introduced into the poroussilica may be insufficient, and thus the polymerization reaction of thecarbon precursor in porous carbon synthesis process may be difficult toproceed. If the amount of the hydrate for introducing the Al acid siteexceeds 1.05 parts by weight, the acid site is rather excessive and thusthe synthesis reaction of the porous carbon may be difficult to proceed.

Step (S2)

In step (S2), porous silica particles may be obtained by evaporating theorganic solvent in the porous silica solution.

By evaporating the organic solvent while stirring the porous silicasolution at room temperature, residual porous silica particles may beobtained.

Step (S3)

In step (S3), the porous silica particles may be subjected to the firstheat treatment to obtain porous silica particles having an Al acid siteintroduced therein.

The Al acid site is located on the surface of silica to inducepolymerization reaction of a carbon precursor such as furfuryl alcoholand plays a role in promoting the synthesis of porous carbon.

The first heat treatment may be performed by raising the temperature to500 to 600° C. at a rate of 0.5° C./min to 3° C./min in an airatmosphere.

If the rate of temperature rise during the first heat treatment is lessthan 0.5° C./min, the time required for heat treatment becomes longer,and thus the physical properties of the porous silica particles may bedenatured. If the rate of temperature rise exceeds 3° C./min, the acidsites may not be formed as much as desired in the porous silicaparticles.

If the temperature of the first heat treatment is less than 500° C., theacid sites may not be formed as much as desired in the porous silicaparticles. If the temperature of the first heat treatment exceeds 600°C., the physical properties of the porous silica particle may bedenatured.

Step (S4)

In step (S4), a carbon-silica composite may be obtained by impregnatingthe porous silica particles having the Al acid site introduced thereinwith a carbon precursor and then subjecting it to the second heattreatment.

At this time, the carbon precursor may be impregnated into the pores ofthe porous silica particles in the form of a solution.

In the present invention, the carbon precursor may be at least oneselected from the group consisting of furfuryl alcohol, sucrose, andglucose.

In the present invention, since the carbon precursor in a liquid phaseis used as the carbon precursor, a separate solvent for dissolving thecarbon precursor may not be required. However, the carbon precursor asthe liquid phase may be further dissolved in the solvent. At this time,the solvent used in the solution of the carbon precursor may betetraethylene glycol dimethyl ether (TEGME).

The solution of the carbon precursor may be prepared by mixing thecarbon precursor and the solvent at a weight ratio of 1:0.5 to 1.5. Ifthe weight ratio of the solvent to the carbon precursor is 1:less than0.5, since the amount of the carbon precursor is relatively high, thewall thickness of the pore may be increased and thus the pore volume ofthe mesoporous carbon which is a product may be reduced. On the otherhand, if the weight ratio is 1:more than 1.5, the amount of carbonprecursor contained in the solution may be small and thus the wallthickness of the pores may be reduced and it may be difficult tomaintain the shape of the mesoporous carbon.

Therefore, the volume of the micropores and mesopores may be controlledby tetraethylene glycol dimethyl ether, a solvent for further dissolvingthe carbon precursor in the liquid phase.

In the present invention, the second heat treatment is a process forinducing polymerization of a carbon precursor wherein a carbon-silicacomposite can be obtained by the second heat treatment.

The temperature of the second heat treatment may be 70° C. to 100° C.,preferably 75° C. to 95° C., more preferably ° C. to 90° C. If thetemperature of the second heat treatment is less than the above range,the polymerization reaction rate of the carbon precursor is not fast oris not properly initiated. If the temperature of the second heattreatment exceeds the above range, the physical properties of the formedcarbon-silica composite may be denatured.

The time period of the second heat treatment may be 7 hours to 10 hours,preferably 7.5 hours to 9.5 hours, more preferably 8 hours to 9 hours.If the time period of the second heat treatment is less than the aboverange, the polymerization reaction of the carbon precursor may not becompletely completed. If the time period of the second heat treatmentexceeds the above range, the excess time period does not significantlyaffect the outcome of the reaction and thus there is no benefit arisingfrom exceeding the time period of heat treatment.

Also, if the heat treatment is performed within the range of thetemperature and the time period of the heat treatment specified as thecondition for the second heat treatment, the uniformity of shape andsize of the porous carbon produced may be improved.

Also, in the present invention, the method may further comprise thethird heat treatment step performed by raising the temperature at a rateof 0.5° C./min to 1° C./min under an inert atmosphere after the secondheat treatment and heat-treating at 700° C. to 1000° C. for 1 hour to 5hours.

The inert atmosphere may be formed by at least one inert gas selectedfrom the group consisting of argon, nitrogen, helium, neon, and krypton.When argon among the inert gases is used, the reaction from which thecarbon-silica composite is formed may be performed more smoothly.Therefore, it may be preferable that the inert atmosphere is formed ofargon among the inert gases.

If the rate of temperature rise during the third heat treatment is lessthan 0.5° C./min, the carbon-silica composite is incompletely formed. Ifthe heating rate exceeds 1° C./min, there may be a problem that affectsthe overall porous structure.

If the temperature of the third heat treatment is less than 700° C., thecarbon-silica composite is incompletely formed. If the temperature ofthe third heat treatment exceeds 1000° C., the physical properties ofthe formed carbon-silica may be denatured.

Also, if the heat treatment is performed within the ranges of thetemperature raising rate, and the temperature and the time period of theheat treatment specified as the condition for the third heat treatment,the uniformity of shape and size of the porous carbon produced may befurther improved.

Step (S5)

In step (S5), porous carbon may be obtained by etching the silica in thecarbon-silica composite.

At this time, the carbon-silica composite may be dispersed in a mixedsolution of an organic solvent and water, and the silica may be etchedusing an etching solution.

Considering the dispersibility of the carbon-silica composite, theorganic solvent and water may be mixed in a weight ratio of 1:0.8 to1.2. The organic solvent may be at least one selected from the groupconsisting of ethanol, methanol, propanol, butanol, ethyl acetate,chloroform and hexane.

The etching solution may be a solution containing at least one selectedfrom the group consisting of hydrofluoric acid (HF), hydrogen peroxide(H₂O₂), nitric acid (HNO₃), potassium hydroxide (KOH) and sodiumhydroxide (NaOH).

Positive Electrode Active Material

The present invention also relates to a positive electrode activematerial comprising a porous carbon; and a sulfur-containing materialcarried within the pores of the porous carbon wherein the positiveelectrode active material may be for a lithium secondary battery.Preferably, the positive electrode active material may be a positiveelectrode active material for a lithium-sulfur secondary battery.

The sulfur-containing active material may be at least one selected fromthe group consisting of elemental sulfur (S₈) and sulfur-basedcompounds. Specifically, the sulfur-based compound may be selected fromLi₂Sn (n≥1), an organic sulfur compound or a carbon-sulfur polymer((C₂S_(x))_(n): 2.5≤x≤50, n≥2).

The content of sulfur may be 50 to 80 wt. %, preferably 65 to 77 wt. %,based on the total weight of the positive electrode active material. Ifthe content of sulfur is less than 50 wt. %, the energy density of thebattery may be lowered. If the amount of sulfur is more than 80 wt. %,the volume expansion of sulfur and low electric conductivity duringcharging/discharging may be problems.

Preparation Method of Positive Electrode Active Material

The present invention also provides a method for preparing a positiveelectrode active material as described above, comprising the steps of:(P1) forming a mixed powder of the porous carbon and sulfur or a sulfurcompound; (P2) mixing the mixed powder with a solvent for dissolvingsulfur to form a mixture; and (P3) subjecting the mixture to heattreatment under vacuum to impregnate the pores of the porous carbon withsulfur.

Hereinafter, a method of preparing a positive electrode active materialaccording to the present invention will be described in more detail foreach step.

Step (P1)

The porous carbon for preparing the positive electrode active materialmay be prepared by the process for preparing the porous carboncomprising the steps (S1) to (S5) as described above. The sulfur maycomprise elemental sulfur (S₈), a sulfur-based compound or a mixturethereof. Specifically, the sulfur-based compound may be Li₂Sn (n≥1), anorganic sulfur compound or a carbon-sulfur polymer ((C₂S_(x))_(n):2.5≤x≤50, n≥2).

The porous carbon and sulfur may be mixed in a powder state to obtain amixed powder. At this time, the porous carbon and sulfur may be mixedsuch that the weight of sulfur is 50 to 80 wt. %, preferably 65 to 77wt. %, based on the total weight of the positive electrode activematerial to be produced.

Step (P2)

The mixed powder obtained in step (P1) is mixed with a solvent to form amixture, wherein the sulfur contained in the mixed powder may bedissolved by using a solvent for dissolving sulfur having a high sulfursolubility as the solvent so that the dissolved liquid sulfur is carriedin the pores of the porous carbon.

At this time, the solvent for dissolving sulfur may be at least oneselected from the group consisting of CS₂ solvent, ethylenediamine,acetone, and ethanol. In particular, when the CS₂ solvent is used, theselective solubility of sulfur contained in the mixed powder is high, sothat it is advantageous to dissolve sulfur to be carried in the insideof pores contained in the porous carbon.

Step (P3)

The mixture formed in step (P2) may be subjected to heat treatment in avacuum to fix the liquid sulfur carried in the pores contained in theporous carbon to the surface of the pores.

A positive electrode active material having a shape, in which sulfur iscarried in the porous carbon, may be prepared by steps (P1) to (P3). Thepositive electrode active material may be applied to a positiveelectrode of a lithium-sulfur secondary battery.

Positive Electrode and Lithium Secondary Battery

The present invention also relates to a positive electrode comprisingthe positive electrode active material as described above and a lithiumsecondary battery comprising said positive electrode.

The positive electrode may comprise a positive electrode currentcollector and a positive electrode active material layer disposed on thepositive electrode current collector and comprising the positiveelectrode active material and optionally a conductive material and abinder.

As the positive electrode current collector, specifically it may bepreferable to use foamed aluminum, foamed nickel, or the like havingexcellent conductivity.

In addition, the positive electrode active material layer may furthercomprise a conductive material for allowing electrons to move smoothlyin the positive electrode together with the positive electrode activematerial, and a binder for enhancing adhesion between the positiveelectrode active materials or between the positive electrode activematerial and the current collector.

The conductive material may be carbon-based materials such as carbonblack, acetylene black, and ketjen black; a conductive polymer such aspolyaniline, polythiophene, polyacetylene, polypyrrole and may becomprised in an amount of 5 to 20 wt. % based on the total weight of thepositive electrode active material layer. If the content of theconductive material is less than 5 wt. %, the effect of improving theconductivity by the use of the conductive material is insignificant. Onthe other hand, if the content of the conductive material exceeds 20 wt.%, the content of the positive electrode active material becomesrelatively small, and thus there is a possibility that the capacitycharacteristics may be deteriorated.

In addition, the binder may be poly(vinyl acetate), polyvinylalcohol,polyethyleneoxide, polyvinylpyrrolidone, alkylated polyethyleneoxide,cross-linked polyethyleneoxide, polyvinylether,poly(methylmethacrylate), polyvinylidene fluoride, copolymer ofpolyhexafluoropropylene and polyvinylidene fluoride (product name:Kynar), poly(ethylacrylate), polytetrafluoroethylene, polyvinylchloride,polyacrylonitrile, polyvinylpyridine, polystyrene, and derivatives,blends and copolymers thereof and the like. Also, the binder may bepreferably comprised in an amount of 5 to 20 wt. % based on the totalweight of the positive electrode active material layer. If the contentof the binder is less than 5 wt. %, the effect of improving the adhesionbetween the positive electrode active materials or between the positiveelectrode active material and the current collector depending on the useof the binder is insufficient. On the other hand, if the content of thebinder exceeds 20 wt. %, the content of the positive electrode activematerial becomes relatively small, and thus there is a possibility thatthe capacity characteristics may be deteriorated.

The positive electrode as described above may be prepared by aconventional method, and specifically, may be prepared by applying thecomposition for forming the positive electrode active material layerprepared by mixing the positive electrode active material, theconductive material and the binder in an organic solvent, on a currentcollector, followed by drying and optionally rolling.

At this time, the organic solvent may be a solvent which may uniformlydisperse the positive electrode active material, the binder, and theconductive material, and which is easily evaporated. Specifically, theorganic solvent may comprise acetonitrile, methanol, ethanol,tetrahydrofuran, water, isopropyl alcohol and the like.

The lithium secondary battery comprising the positive electrodecomprising the porous carbon according to the present invention mayincrease the density of the positive electrode because the porous carbonhas a uniform size and shape. Preferably, the lithium secondary batterymay be a lithium-sulfur secondary battery.

In addition, when compared with activated carbon having only micropores,since the porous carbon has micropores, thereby having a high specificsurface area and also has mesopores, thereby having a large pore volume,the pores are not clogged even after the impregnation of sulfur, andthus the electrolyte solution is easy to enter and exit, and as aresult, the discharging capacity and the output characteristic areexcellent.

Hereinafter, preferred examples will be presented to facilitateunderstanding of the present invention. It should be understood,however, that the following examples are illustrative of the presentinvention and that various changes and modifications may be made withinthe scope and spirit of the present invention, and it is obvious thatsuch changes and modifications are within the scope of the appendedclaims.

Preparation Example 1: Synthesis of Porous Silica

8.0 g of a triple block copolymer EO₂₀PO₇₀EO₂₀ (trade name: PluronicP123, EO: ethylene glycol, PO: propylene glycol), 10 g of potassiumchloride (KCl) and 20 mL of 37.2 wt. % hydrochloric acid (HCl) weremixed with 130 mL of water and 10 mL of ethanol, and the mixture wasstirred at room temperature for 8 hours or more. Next, when PluronicP123 was completely dissolved, 9.26 mL of mesitylene was added andstirred at 40° C. for 2 hours.

18.4 mL of tetraethylorthosilicate (TEOS), a silica source, was addedand stirred vigorously at the same temperature for 2 minutes. The mixedsolution was kept at the same temperature for 20 hours. Thereafter,0.092 g of ammonium fluoride was added to the mixed solution, stirredvigorously for 2 minutes, and then hydrothermally synthesized in an ovenat 100° C. for 24 hours. Then, the mixture was filtered with a mixtureof ethanol and water, dried at room temperature, and heat-treated at550° C. for 4 hours in an air atmosphere to finally synthesize poroussilica.

FIGS. 1a and 1b are SEM (scanning electron microscope) photographs ofporous silica synthesized in Preparation Example 1.

Referring to FIG. 1a , it can be seen that a porous silica having aspherical particle shape and mesopore was produced.

In addition, FIG. 1B is an enlarged photograph of FIG. 1a . It can beseen that the diameter of the prepared porous silica is 5 μm, andmesopore is well developed.

Example 1: Preparation of Porous Carbon, Positive Electrode ActiveMaterial, Positive Electrode and Lithium-Sulfur Secondary Battery

(1) Preparation of Porous Carbon

1 g of spherical porous silica prepared in Preparation Example 1 wasuniformly dispersed in 50 ml of ethanol and 0.21 g of aluminum chloridehexahydrate was mixed together and stirred for 2 hours to obtain aporous silica solution.

While the porous silica solution was stirred at room temperature, all ofthe solvent ethanol was evaporated.

Thereafter, the remaining powder of porous silica particles wascollected, heated at 1° C./min in an air atmosphere to raise thetemperature to 550 □ and thus subjected to the first heat treatment andmaintained for 5 hours.

After the first heat treatment, the pore volume of the porous silicawith the introduced Al acid site was measured, furfuryl alcohol as muchas 1/2 volume of the measured pore volume and tetraethylene glycoldimethyl ether (TEGDME) as much as 1/2 volume of the measured porevolume were mixed and impregnated using a vacuum. That is, the weightratio of furfuryl alcohol as a carbon precursor and tetraethylene glycoldimethyl ether as a solvent is 1:1.

After that, the mixture was maintained in an oven at 80 □ for 8 hours toperform the second heat treatment to induce polymerization of furfurylalcohol.

In addition, the temperature was raised at a rate of 1 □/min in an Aratmosphere to raise the temperature to 850 □ and maintained for 3 hoursto prepare a carbon-silica composite.

The carbon silica composite was dispersed in a mixed solution of ethanoland water at a weight ratio of 1:1, and silica was etched with HF toprepare porous carbon.

(2) Preparation of Positive Electrode Active Material

The prepared porous carbon was mixed with sulfur powder to obtain amixed powder. At this time, the mixed powder was prepared so that theweight of sulfur in the positive electrode active material to beproduced could be 70 wt. %.

Sulfur contained in the mixed powder was dissolved while dropping CS2 asa solvent for dissolving sulfur into the mixed powder.

The mixed powder in which sulfur was dissolved was kept under vacuum for30 minutes and then held at 155 □ for 12 hours to fix the dissolvedsulfur to the inner pores of the porous carbon to produce a positiveelectrode active material.

(3) Manufacture of Lithium-Sulfur Secondary Battery

A positive electrode mixture of 80 wt. % of the positive electrodeactive material, 10 wt. % of carbon black (conductive material) and 10wt. % of PVDF (binder) was added to N-methyl-2-pyrrolidone (NMP) as asolvent to prepare a positive electrode slurry, and then the slurry wascoated on the current collector of the aluminum foil to prepare apositive electrode. At this time, the content of sulfur in the positiveelectrode is 2.3 mg/d.

A lithium-sulfur secondary battery was manufactured by using the lithiumfoil with a thickness of 200 μm as a negative electrode, an organicsolution obtained by dissolving 2 wt. % LiNO₃ additive in 1M LiTFSI(DME/DOL, 1:1 volume ratio) as the electrolyte solution, and thepolypropylene film as a separator.

-   -   LiTFSI: Bis(trifluoromethane) sulfonamide lithium salt    -   DME: Dimethoxymethane    -   DOL: 1,3-dioxolane

Example 2

The same procedure as in Example 1 was carried out, except that afterthe first heat treatment, furfuryl alcohol and tetraethylene glycoldimethyl ether in a weight ratio of 1:0.67 were impregnated into thepores of the porous silica with the introduced Al acid site, and thepore volume of the porous silica was measured and then impregnated asmuch as the measured pore volume.

Example 3

The same procedure as in Example 1 was carried out, except that afterthe first heat treatment, furfuryl alcohol and tetraethylene glycoldimethyl ether in a weight ratio of 1:0.43 were impregnated into thepores of the porous silica with the introduced Al acid site, and thepore volume of the porous silica was measured and then impregnated asmuch as the measured pore volume.

Example 4

The same procedure as in Example 1 was carried out, except that afterthe first heat treatment, furfuryl alcohol and tetraethylene glycoldimethyl ether in a weight ratio of 1:0.25 were impregnated into thepores of the porous silica with the introduced Al acid site, and thepore volume of the porous silica was measured and then impregnated asmuch as the measured pore volume.

Example 5

The same procedure as in Example 1 was carried out, except that afterthe first heat treatment, furfuryl alcohol and tetraethylene glycoldimethyl ether in a weight ratio of 1:2.33 were impregnated into thepores of the porous silica with the introduced Al acid site, and thepore volume of the porous silica was measured and then impregnated asmuch as the measured pore volume.

Comparative Example 1: Activated Carbon

Activated carbon (MSP-20, Kanto chemical Co.) containing microporesalone was prepared.

The same procedure as in Example 1 was carried out, except that apositive electrode active material, a positive electrode, and alithium-sulfur secondary battery were manufactured using the activatedcarbon instead of the porous carbon.

Experimental Example 1: Observation of Porous Carbon

The shape, size and pore of the porous carbon prepared in Example 1 wereobserved.

FIGS. 2a to 2c are SEM (scanning electron microscope) and TEM(transmission electron microscope) photographs of the porous silicaprepared in Preparation Example 1.

FIG. 2a is a SEM photograph of the porous carbon prepared in Example 1,which shows that spherical porous carbon was overall synthesized.

FIG. 2b is an enlarged SEM photograph of the porous carbon prepared inExample 1, which shows that the particle size of the spherical porouscarbon is 5 μm.

FIG. 2c is a TEM photograph of the porous carbon prepared in Example 1,which shows that a well-developed porous carbon with mesopores wassynthesized.

Experimental Example 2: Measurement of Physical Properties of PorousCarbon and Activated Carbon

In order to compare the surface area, pore volume and pore size of theporous carbon of Example 1 and the activated carbon of ComparativeExample 1, nitrogen adsorption/desorption experiments were carried out.

Nitrogen adsorption/desorption experiments were carried out using theequipment (Tristar II 3020, Micromeritics) for measuring specificsurface area. Specifically, the sample to be analyzed (about 50 mg to100 mg) was placed in a glass tube for analysis and a pretreatmentprocess was performed by removing the moisture adsorbed in the pores ofthe sample to be analyzed for 8 hours at 100 □ in a vacuum state.Nitrogen adsorption/desorption analysis was performed on the pretreatedsample while flowing nitrogen gas using liquid nitrogen.

FIGS. 3a to 3d are graphs of the isotherm linear plot and the porediameter distribution as a result of nitrogen adsorption/desorptionanalysis of the porous carbon according to Examples 1 to 4,respectively, and FIG. 3e are SEM (scanning electron microscope) and TEM(transmission electron microscope) photographs of the porous silicaaccording to Example 5.

Referring to FIGS. 3a to 3e , it can be seen that the specific surfacearea and pore volume of the porous carbon of Examples 1 to 3 arerelatively good, and in the case of Example 5, the weight ratio offurfuryl alcohol as a carbon precursor and tetraethylene glycol dimethylether as a solvent is so high that porous carbon may not maintain itsspherical shape.

FIG. 4 is a graph showing the results of nitrogen adsorption/desorptionanalysis on the porous carbon of Example 1 and the activated carbon ofComparative Example 1.

Referring to FIG. 4 and Table 1 below, it can be seen that the porouscarbon of Example 1 contains both micropores and mesopores, and thus thespecific surface area is slightly smaller than the activated carbon ofComparative Example 1 but does not differ greatly, and the pore volumeis about 4 times that of the activated carbon.

TABLE 1 Carbon precursor:solvent Specific (furfuryl surface Pore Porealcohol:TEGDME) area volume diameter Example 1 1:1   1,267 m²/g 3.7cm³/g 4 nm, 30 nm Example 2 1:0.67 1,147 m²/g 3.4 cm³/g 4 nm, 21 nmExample 3 1:0.43 1,022 m²/g 2.5 cm³/g 3.8 nm, 29 nm Example 4 1:0.251,013 m²/g 2.1 cm³/g 3.8 nm, 31 nm Example 5 1:2.33 Impossible tomaintain spherical shape Comparative — 1,943 m²/g 0.9 cm³/g 2 nm orExample 1 less

Experimental Example 2: Experiment of Discharging Capacity

The discharging capacity of the lithium-sulfur secondary batterycontaining the porous carbon of Example 1 and the activated carbon ofComparative Example 1 was analyzed in the first cycle ofcharging/discharging by analyzing the voltage profile. The dischargingcapacity was determined by a constant current test (galvanostatic test)at 1 C rate of 1672 mA/g and a constant current test at 0.2 C rate.

FIG. 5 is a graph showing voltage profiles depending on capacities inlithium-sulfur secondary batteries of Example 1 and Comparative Example1.

Referring to FIG. 5, the porous carbon of Example 1 is different fromthe activated carbon of Comparative Example 1 in that it contains porouscarbon with the formed mesopores, and it can be seen that the porouscarbon of Example 1 has a relatively small specific surface area, butthe pore volume is relatively large due to the presence of mesopores,and as a result, the discharge capacity may be increased due to the factthat sulfur may be efficiently carried in pores.

Experimental Example 3: Life Test of Battery

In order to confirm whether or not the life characteristics of thelithium-sulfur secondary battery were improved by mitigating theleaching issue of poly sulfide in electrochemical reduction reaction ofsulfur by the porous carbon containing micropores and mesopores andhaving a uniform shape and size and whether or not the efficiency ofcarrying sulfur was improved due to the increased pore volume and thereversible capacity was improved, galvanostatic charging-discharginganalysis was performed. The galvanostatic charging-discharging analysiswas carried out at 0.2 C in 1.7 V to 3.0 V voltage range (vs Li/Li⁺) (1C: 1672 mA/g).

FIG. 6 is a graph showing the results of galvanostaticcharging-discharging analysis for lithium-sulfur secondary batteries ofExample 1 and Comparative Example 1.

Referring to FIG. 6, it can be seen that the lithium-sulfur secondarybattery of Example 1, which comprises the positive electrode materialcontaining the porous carbon containing micropores and mesopores, issuperior in battery life to Comparative Example 1 and has an additionalcapacity of about 200 mAh/g.

Experimental Example 4: Experiment of High Rate Characteristics

High rate characteristics of the lithium-sulfur secondary battery inExample 1 and Comparative Example 1 were experimented by cycling 5 timesfor each density while sequentially changing the charging/dischargingdensities to 0.1 C, 0.2 C, 0.5 C, 1 C and 5 C.

FIG. 7 is a graph showing discharging capacity of the lithium-sulfursecondary batteries of Example 1 and Comparative Example 1 depending onthe number of cycles during high-rate discharging.

Referring to FIG. 7, it was confirmed that Example 1 shows higherretention capacity at high rate discharging than Comparative Example 1,and that since the porous carbon of Example 1 comprises mesopores, therapid entry and exit of the electrolyte solution is possible, thegeneration and decomposition of the discharging product in the pore isperformed based on the high pore volume, and thus the leaching of polysulfide was controlled and the capacity expression at high rate wasexcellent.

Although the present invention has been described with reference to theexemplary embodiments and drawings, it is to be understood that thepresent invention is not limited thereto and that various modificationsand variations are possible within the scope of the technical idea ofthe present invention and the equivalents of the claims to be describedbelow by a person having ordinary skill in the art to which the presentinvention pertains.

1. A positive electrode for a lithium secondary battery comprisingporous carbon particles, wherein the porous carbon particles comprisemicropores having a diameter of 1 nm to 8 nm and mesopores having adiameter of 2 nm to 50 nm, wherein the porous carbon particles arespherical particles having a particle diameter of 2 μm to 10 μm, andwherein the porous carbon comprises the micropores and the mesopores ina pore volume ratio of 1:40 to
 50. 2. The positive electrode accordingto claim 1, wherein the porous carbon particles comprise microporeshaving a diameter of 0.1 nm to 1.8 nm.
 3. The positive electrodeaccording to claim 1, wherein the porous carbon particles comprisemicropores having a diameter of 0.3 nm to 1.5 nm.
 4. The positiveelectrode according to claim 1, wherein the porous carbon particlescomprise mesopores having a diameter of 10 nm to 50 nm.
 5. The positiveelectrode according to claim 1, wherein the porous carbon particlescomprise mesopores having a diameter of 30 nm to 50 nm.
 6. The positiveelectrode according to claim 1, wherein the porous carbon particlescomprise mesopores having a diameter of 20 nm to 40 nm.
 7. The positiveelectrode according to claim 1, wherein the porous carbon particles arespherical particles having a particle diameter of 3 μm to 7 μm.
 8. Thepositive electrode according to claim 1, wherein the porous carbonparticles are spherical particles having a particle diameter of 4 μm to6 μm.
 9. The positive electrode according to claim 1, wherein the porevolume of mesopores is 3.5 cm²/g or more.
 10. The positive electrodeaccording to claim 1, wherein the pore volume of mesopores is 3.5 cm²/gto 4.5 cm²/g.
 11. The positive electrode according to claim 1, whereinthe pore volume of mesopores is 3.8 cm²/g to 4.2 cm²/g.
 12. The positiveelectrode according to claim 1, wherein the specific surface area of theporous carbon is 1000 m²/g to 1300 m²/g.
 13. The positive electrodeaccording to claim 1, wherein the specific surface area of the porouscarbon is 1150 m²/g to 1300 m²/g.
 14. The positive electrode accordingto claim 1, wherein the specific surface area of the porous carbon is1200 m²/g to 1300 m²/g.
 15. A lithium secondary battery comprising thepositive electrode according to claim 1.